Vampires are not capable of resisting their natures forever all the time. Self control for humans and for vampires is a
limited resource. Controlling themselves to NOT do what is in their nature to do, is extremely difficult. They will sometimes even act without thinking, or unconsciously. The bloodlust is a like a food craving, or an itch that wants to be scratched. Eventually it gets so intense and so annoying you will do anything to satisfy it. Stuff like Type O helps prolong the period where they can resist, but its like having a craving for a food, and eating anything BUT what you really want. And the craving never goes away, and you are never really satisfied.
It should be noted that vampires in the shifters universe DO NOT need to kill. It feels amazing, but they don't have to do it. If they are able to secure donors and take proper precautions, its not unsafe for them to feed directly from people. There are enough vampire wanna-be types (posers, fangirls/boys, gothy wannabees, just look up vampire subculture stuff) that there are people perfectly happy and willing to donate fresh blood and find it erotic and pleasurable to do so. They simply have to avoid direct blood to blood contact, most avoid mouth to wound contact through either cutting or using medical tubing and needles to extract blood.
Vampires in Shifters are not fluffy happy vegetarian vampires. They don't sparkle. They can't resist the need to feed on blood forever, and most don't care to. They don't even see humans as equals. They are like cattle to a vampire for the most part. They only care really about the repercussions making things inconvenient for them.